Support reader conditional when reading/merging data_readers

[alan-ghelardi]

• Please check if the PR fulfills these requirements

• There is an open issue describing the problem that this pr intents to solve.
  • You have a descriptive commit message with a short title (first line).
• Tests for the changes have been added (for bug fixes / features).
• Docs have been added / updated (for bug fixes / features).

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Bug fix

• What is the current behavior?

When Vessel encounters a data_reader file containing reader conditionals, a
runtime exception (no dispatch macro for ?) is thrown.

• What is the new behavior (if this is a feature change)?

As of this pull request Vessel reads data_readers files containing reader
conditionals properly.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No

• Other information:

[alan-ghelardi]

@hlship do you mind to take a look at this pull request? People reported this issue during the build of their services. In fact, I believe that there is no impact on the feature you implemented recently, but I'd appreciate your comments if I am not paying attention to something.